The present invention relates to the field of oil well completion and formation testing and, more particularly, to an improved method and apparatus for determining the shut-in pressure of a hydrocarbon formation. Still more particularly, the present invention includes a method and apparatus for measuring the shut-in pressure of a hydrocarbon formation perforated by a perforating gun suspended from a tubing string.
One method for testing a formation in a cased well includes running an electric line casing gun perforator in mud of sufficient weight to control the well pressure, perforating the casing adjacent to the zone to be tested, and then withdrawing the perforating gun. Test tools are then run into the well on a pipe string with well pressure being controlled with casing fluid of appropriate weight. A packer is set to close the annulus and a valve is opened in the pipe string to permit fluids from the formation to flow through the pipe string to the surface.
A second method for testing a formation includes running a tool string on drill pipe into the cased borehole with the tool string including full opening test tools with a full opening valve, and a packer disposed on the tool string for packing off the annulus. The casing adjacent to the zone to be tested is packed off with the packer and the full opening valve is then opened, providing fluid communication between the flow bore of the pipe string and the lower packed off portion of the casing. A small through-tubing perforating gun is lowered on an electric line through the test tools, and the casing adjacent the zone is perforated. The wireline perforating gun is then lubricated out of the well.
Another formation testing method is disclosed in the U.S. Pat. No. 2,169,559. In the '559 patent, a formation tester, sub, packer, perforated pipe, perforating gun, and bull plug are all suspended on the end of a drill pipe string. The formation tester includes a limited opening valve and a mandrel for opening the valve. A number of passageways in the sub permit fluid flow from a point beneath the sub into the formation tester. The bull plug below the perforating gun may include a pressure recording apparatus. In operation, the packer is set to seal the lower portion of the well from the portion above the packer and the drill pipe is rotated and lowered causing the mandrel to open the valve in the formation tester. This actuates the perforating gun which detonates and perforates the formation. Any fluid in the formation then flows through the perforations and through the perforated pipe above the perforating gun. This fluid must then pass through the limited openings of the passageways in the sub and of the valve and into the drill pipe. After a sufficient length of time, the drill pipe is lifted to allow the valve to close. When the valve closes, a sample of the fluid from the formation is entrapped in the drill pipe. The packer is then released and the entire assembly is removed from the well with the entrapped sample.
Various drill stem test procedures may be used in determining the potential productivity of a subsurface hydrocarbon formation which has previously been perforated. The typical procedure begins by including in the drill string various test apparatus. After the packer is set to seal the casing annulus below the packer, the well is perforated and the formation is then permitted to produce through the drill string to provide an indication of the ability of the formation to produce without the use of enhancement techniques.
After a specified time interval, a tester valve positioned in the drill string above the packer is closed, thereby closing-in the hydrocarbon formation. Pressure measuring apparatus within the closed-in portion of the drill string records the rate of pressure build-up and the ultimate shut-in pressure of the hydrocarbon formation. This data also provides an accurate basis for evaluating the hydrocarbon formation, including, for example, the well's production capability, transmissibility, actual flow capacity, permeability, and formation damage. After the test sequence is completed, pressure above and below the packer is equalized, the packer is unseated, formation fluid is flushed from the drill string by reverse circulation, and the drill pipe is pulled from the well. The drill stem test procedure is further described in II Composite Catalog of Oil Field Equipment & Services, 3286-88 (1978-1979) and U.S. Pat. No. 3,970,147 to Jessup et al.
Well completion is typically accomplished by running a small through-tubing perforating gun on a wire-line through the tubing string suspended and packed off in the cased borehole. The borehole is filled with drilling mud or some other appropriate fluid so as to prevent the blow-out of the perforating gun upon detonation. However, this drilling fluid also prevents the flow of hydrocarbon fluids into the borehole at the time of perforation. Generally, the through-tubing perforating gun is actuated electrically. After the through-tubing gun is lubricated out of the well, the control fluid in the tubing string is removed to bring in the well.
Another method for completing oil and/or gas wells, now well known in the art, includes lowering into the cased borehole and tubing string and perforating the well by shooting perforations through the casing and cement into the hydrocarbon formation to permit the hydrocarbons to flow into the cased borehole and up to the surface. U.S. Pat. No. 3,706,344 to Vann discloses the method of suspending a packer and perforating gun on a tubing string, setting the packer to isolate the production zone, releasing the trapped pressure below the packer by opening the tubing string to fluid flow, actuating the perforating gun through the tubing string, and immediately producing the well through the tubing string upon perforation. One means for actuating the perforating gun includes dropping a bar through the tubing string to impact the firing head of the perforating gun.
Vann's completion technique exhibits several advantages, as are described in the above-named patent, over prior art completion techniques. Vann's technique does not, however, provide for any particular means for evaluating the potential of the formation. Use of the drill stem test procedure with Vann's technique would require unseating the packer and pulling the entire tubing string from the borehole, thereby defeating one of the advantages of the Vann method. Hence, it would be useful to devise a method and apparatus for evaluating and testing a hydrocarbon formation, the method and apparatus being particularly useful in conjunction with a formation testing or well completion method where the perforating gun is suspended on the tubing string.